This invention relates to articles having optical coatings and a method for their preparation. More particularly, it relates to the provision of optically coated articles having a thermally stable optical response.
Optically coated articles are useful as interference filters in such applications as thermal-optical switching. They generally comprise a substrate having a number of coatings, the substrate and coatings being transparent to light in various regions of the spectrum, typically the visible and/or short wavelength infrared regions, wherein the coatings having alternating high and low refractive indices. The term xe2x80x9cshort wavelength infraredxe2x80x9d refers to the region in the range of about 700-2,500 nm. An essential feature of the coating system is a substantial difference between the refractive indices of adjacent coatings, typically represented by a ratio of refractive indices (hereinafter xe2x80x9cRxe2x80x9d) of at least about 1.5. Higher ratios are often desirable.
Typical optically coated articles known in the art comprise a glass substrate and alternate coatings of silicon oxide (SiO2) and either tantalum oxide (Ta2O5) or titanium dioxide (TiO2). The coating systems of such articles have R values of 1.5 and 1.6, respectively.
It would be desirable to produce optical articles in which the coating systems have a higher R value, for example above 2.0. Such values can be obtained by employing alternate coatings of silicon dioxide and amorphous hydrogenated silicon (hereinafter referred to as xe2x80x9ca-Si:Hxe2x80x9d) on a fused silica (i.e., glass) substrate. The coatings may conveniently be deposited by plasma enhanced chemical vapor deposition (hereinafter referred to as xe2x80x9cPECVDxe2x80x9d). It has been found that a SiO2/a-SiH coating system consisting of 46 coating layers and having a total thickness of about 7.5 microns exhibits optical behavior equivalent to a SiO2xe2x80x94Ta2O5 coating system consisting of 64 layers and having a thickness of 13 microns.
The optical response of an optical article depends on the refractive index and thickness of the coating deposited thereon. Both of these properties are a function of temperature. The thermo-optic coefficient describes the change of the material refractive index as a function of temperature; the thermo-mechanical coefficient, also known as the coefficient of thermal expansion (hereinafter referred to as xe2x80x9cCTExe2x80x9d), describes physical dimension changes with temperature.
In the case of glass coated with alternating layers of SiO2 and a-Si:H, there is a profound change in optical behavior with temperature based on the CTE of the substrate and the CTE and thermo-optic coefficient of the coatings. A principal factor in this change is the relatively high thermo-optic coefficient of the a-Si:H layer. Upon heating, therefore, a shift of the interference structure, usually by the center wavelength between the full width at half-maximum (FWHM) of reflected light, is observed. The shift is typically on the order of 7 nm over a 75xc2x0 C. variation in temperature. The substrate CTE in some cases can be used to compensate the optical shift of the coatings, as just described.
Glass has a CTE on the order of 7.8 ppm/xc2x0 C. Temperature-related effects on optical behavior (as exemplified by the aforementioned wavelength shift) will be minimized if a substrate is chosen such that its CTE is as close as possible to 66 ppm/xc2x0 C., whereupon the substrate CTE will essentially cancel out the effects of the coatings. This would be highly desirable.
It is therefore of interest to provide an optical article in which temperature-induced changes in optical performance are minimized.
The present invention provides optical articles in which temperature-induced variations in optical performance are essentially negligible. A method of producing such articles is also provided. The basis for the invention is the discovery of the effect of coating a resinous substrate having a CTE on the order of 70 ppm/xc2x0 C.
In one of its aspects, the invention is an optically coated article comprising a polymeric substrate and a plurality of coating layers, the plurality of layers comprising alternate layers of silicon dioxide and amorphous hydrogenated silicon, said article being transparent to light in the short wavelength infrared region of the spectrum.
Another aspect of the invention is a method for producing an optically coated article which comprises depositing alternate coating layers of silicon dioxide and amorphous hydrogenated silicon on a thermoplastic resin substrate, the substrate being chosen so that said article is transparent to light in the short wavelength infrared region of the spectrum.